Aromatic pitches such as coal tar pitch or petroleum pitch are composed of a complex mixture of alkyl substituted polycondensed aromatics of high molecular weight and a high degree of aromatic ring condensation. These heavy aromatic products may further be characterized as having a softening point of 100.degree. to 130.degree. C., and high viscosity of 1000-5000 cst at 160.degree. C.
The important feature of pitch is that it can be transformed into a high strength carbon product on melting or carbonization. The microstructure of the carbon product produced is very much dependent on the type of pitch used, and may vary from a highly anisotropic structure having an ordered or crystalline structure, to an unordered or random isotropic structure. The anisotropic structure pitch is preferred for the production of carbon products such as carbon fiber or needle coke.
Many types of pitches can be produced by varying the aromatic feedstock materials and the processes used in pitch manufacture. One simple method to characterize these pitches is by the use of solvent analysis, for example, the degree of insolubility in benzene, toluene, pyridine, quinoline, or anthracene. For the purpose of the present invention, aromatic pitches are characterized by their insolubilities in toluene and quinoline.
Solvent analysis is a method which is universally used to define the type and composition of various pitches; the quantitative determination of insolubles in toluene and quinoline are two analytical protocols which have become standard in the industry. These insolubles represent the two major fractions of a pitch varying in aromaticity, degree of aromatic ring condensation, and coking characteristics. The different insoluble fractions of a pitch also differ in their physical characteristics such as melting, softening, and viscosity which is a critical requirement for carbon product manufacturing.
The solubility analysis for determining quinoline insolubles is conducted according to ASTM D2318-66 protocol; the solubility analysis for determining toluene insolubilities is conducted by mixing 40 grams of a sample in 320 ml of toluene over an 18-hour time period, filtering, washing the insolubles in additional toluene, drying, and calculating the yield of insolubles as a percentage of initial sample.
The most common feedstocks used for this production of pitches are the heavy aromatic residues obtained from coal carbonization, steam cracking, or the catalytic cracking processes of low molecular weight paraffinic hydrocarbons.
The production of a highly aromatic pitch which will yield a non-ordered, isotropic carbon has previously been described in U.S. Pat. No. 3,721,658. More particularly, this patent describes a process for preparing an aromatic pitch by the catalytic air oxidation of an aromatic feedstock such as steam-cracked tar, at a temperature of 240.degree.-260.degree. C. This pitch has low toluene insolubles (about 15%) and very low quinoline insolubles. Because of the chemical structure of the pitch, and not because of the toluene or quinoline insolubles content, this pitch on melting or carbonizing will yield a highly isotropic carbon.
Another example of aromatic pitch production may be found in U.S. Pat. No. 4,086,156. This patent describes a method for preparing an aromatic pitch by the thermal treatment of steam cracker tar in the absence of oxygen and at a temperature of 380.degree.-390.degree. C. The pitch obtained by this method contains a low concentration of toluene and quinoline insolubles and will produce an isotropic carbon on melting or carbonizing.
Unfortunately, the pitch produced from steam cracked tar is not very suitable for producing anistropic products. Only the pitches produced from catalytic cracking residue feedstocks have been found to be suitable. Examples of these suitable aromatic pitches are described in U.S. Pat. No. 4,219,404.
The residue obtained from the catalytic cracking processes of low molecular weight paraffinic hydrocarbons have been found to have the preferred physical and chemical characteristics for producing the pitch needed in the manufacture of anisotropic carbon products. More particularly, the typical physical and chemical characteristics of a suitable catalytic cracking residue feedstock material is presented in Table I.
TABLE 1 ______________________________________ Characteristics of Catalytic Cracking Residue ______________________________________ Physical Characteristics Viscosity cst at 210.degree. C. 1.0-10.0 Ash Content, wt. % 0.010-2.0 Coking Value (wt. % at 550.degree. C.) 6.0-10.0 Asphaltene; (n-heptane insolubles), % 0.1-12.0 Toluene Insolubles, % 0.010-1.0 Number Average Mol. wt. 220-290 Elemental Analysis Carbon, % 88.0-90.32 Hydrogen, % 7.74-7.40 Oxygen, % 0.10-0.30 Sulfur, % 1.0-4.5 Carbon/Hydrogen Atomic Ratio 0.90-1.0 Chemical Analysis (by Carbon-13 Nuclear Magnetic Resonance Spectroscopy) Aromatic Carbon (atom %) 55-75 Aromatic Ring Distribution (by Mass Spectroscopy) 1 Ring (%) 1.2 2 Rings (%) 23.6 3 Rings (%) 37.5 4 Rings (%) 31.8 5 Rings (%) 3.8 6 Rings (%) 0.9 Molecular Weight Distribution (by Mass Spectroscopy) 175-200 (%) 2.9 220-225 (%) 13.4 225-250 (%) 29.5 250-275 (%) 23.1 275-300 (%) 15.5 300-325 (%) 6.8 325-350 (%) 3.5 Composition (by Clay-Silica Gel Chromatography) Aromatic, % 62.2 Saturate, % 17.0 Polar, % 18.3 ______________________________________
Petroleum pitches, though complex in their chemical structure, can be characterized using advanced modern analytical techniques. For example, we can determine quantitatively the various protons (aromatic, benzylic, aliphatic and naphthenic) present in a pitch by using a proton Nuclear Magnetic Resonance Spectroscopy (P-NMR). We can also determine quantitatively the various types of carbon atoms present in the pitch (aromatic carbon, benzylic carbon and paraffinic carbon) by using a carbon-NMR. We can also determine their molecular weight distribution by using a high temperature gel permeation chromatography. Another important chemical characteristic is the carbon/hydrogen atomic ratio which can be calculated from the carbon and the hydrogen elemental analysis.
The process described in this patent deals with the production of a highly anisotropic aromatic pitch derived from catalytic cracking residue which has a high content of the desired fraction of toluene insolubles and a low level of quinoline insolubles, the toluene insoluble fraction having a low melting point and low viscosity. Our process also deals with the extraction of the toluene fraction from the pitch by solvent extraction.
This preferred fraction in the pitch corresponding to approximately the toluene insolubles have been found to produce highly anisotropic carbon products (e.g. needle coke and carbon fiber) at elevated temperatures. For example, the pitch can be used to produce a carbon fiber by spinning at elevated temperature and pressure. This toluene insolubles fraction of the pitch has very high optical activity (as seen by polarized light microscopy), low melting point and low viscosity and it is considered to be part of liquid crystal (or mesophase) in the pitch. The quinoline insoluble fraction in the pitch was found to have a very high melting point (350.degree.-450.degree. C.) and high viscosity hampering subsequent manufacturing of carbon products and thus is regarded as undesirable when present in the pitch in substantial quantities (e.g. over 5% by weight).
When a pitch is produced in a conventional manner from catalytic cracking residue which contains only the toluene insolubles fraction there is a major problem. When thermally treating the aromatic feed into a pitch, the toluene insolubes fraction start forming in the pitch initially, when the toluene insolubles level in the pitch reaches a certain level, the quinoline insolubles start forming in the pitch, presumably from the further condensation of the toluene insolubles.
We have discovered a process where a petroleum pitch with a high toluene insolubles content can be produced without excessive quinoline insoluble formation. This process takes advantage of the variation in the rate of formation of the toluene and quinoline insolubles which can be varied by varying the thermal treatment temperature and time.